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Zhihua Xie - One of the best experts on this subject based on the ideXlab platform.
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numerical investigation of flow induced rotary oscillation of circular cylinder with rigid splitter plate
Physics of Fluids, 2016Co-Authors: Xiaoling Guo, Guoqiang Tang, Mingming Liu, Chuanqi Chen, Zhihua XieAbstract:Numerical results of fluid flow over a rotationally oscillating circular cylinder with splitter plate are presented here. Different from the previous examinations with freely rotatable assembly, the fluid and structure interactions are treated as a coupled dynamic system by fully considering the structural inertia, stiffness, and damping. The hydrodynamic characteristics are examined in terms of Reduced Velocity Ur at a relatively low Reynolds number Re = 100 for different plate lengths of L/D = 0.5, 1.0, and 1.5, where Ur = U/(Dfn), Re = UD/υ and fn = (κ/J)0.5/2π with U the free stream Velocity, D the diameter of the circular cylinder, υ the fluid kinematic viscosity, fn the natural frequency, J the inertial moment, κ the torsional stiffness, and L the plate length. Contrast to the freely rotating cylinder/plate body, that is, in the limit of κ → 0 or Ur →∞, remarkable rotary oscillation is observed at relatively low Reduced velocities. For the typical case with L/D = 1.0, the maximum amplitude may reach...
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numerical investigation of flow induced rotary oscillation of circular cylinder with rigid splitter plate
Physics of Fluids, 2016Co-Authors: Xiaoling Guo, Guoqiang Tang, Mingming Liu, Chuanqi Chen, Zhihua XieAbstract:Numerical results of fluid flow over a rotationally oscillating circular cylinder with splitter plate are presented here. Different from the previous examinations with freely rotatable assembly, the fluid and structure interactions are treated as a coupled dynamic system by fully considering the structural inertia, stiffness, and damping. The hydrodynamic characteristics are examined in terms of Reduced Velocity Ur at a relatively low Reynolds number Re = 100 for different plate lengths of L/D = 0.5, 1.0, and 1.5, where Ur = U/(Dfn), Re = UD/υ and fn = (κ/J)0.5/2π with U the free stream Velocity, D the diameter of the circular cylinder, υ the fluid kinematic viscosity, fn the natural frequency, J the inertial moment, κ the torsional stiffness, and L the plate length. Contrast to the freely rotating cylinder/plate body, that is, in the limit of κ → 0 or Ur →∞, remarkable rotary oscillation is observed at relatively low Reduced velocities. For the typical case with L/D = 1.0, the maximum amplitude may reach five times that at the highest Reduced Velocity of Ur = 15.0 considered in this work. At the critical Reduced Velocity Ur = 4.2, notable hydrodynamic jumps are identified for the rotation amplitude, response frequency, mean drag coefficient, lift amplitude, and vortex shedding frequency. Moreover, the phase angle between the fluid moment and rotary oscillation abruptly changes from 0 to π at Ur = 6.5. Due to the combined effect of fluid moment, rotation response, and phase difference, the natural frequency of the rotating body varies in flow, leading to a wide regime of lock-in/synchronization (Ur ≥4.2, for L/D = 1.0). The phenomenon of rotation bifurcation, i.e., the equilibrium position of the rotary oscillation deflects to a position which is not parallel to the free stream, is found to only occur at higher Reduced velocities. The longer splitter plate has the lower critical Reduced Velocity. The occurrence of bifurcation is attributed to the anti-symmetry breaking of the wake flow evolution. The resultant asymmetric mean pressure distribution on the splitter plate gives rise to the net lift force and the deviated moment on the assembly, leading to the offset mean position of splitter plate. The global vortex shedding is identified to be the classic 2S mode for both cases with and without the bifurcation, although the second vortex formation and the shedding pattern in the near wake for the bifurcate case are different from the non-bifurcate case with lower Reduced velocities.
Ming Zhao - One of the best experts on this subject based on the ideXlab platform.
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three dimensional numerical investigation of vortex induced vibration of a rotating circular cylinder in uniform flow
Physics of Fluids, 2018Co-Authors: Adnan Munir, Ming Zhao, Dezhi NingAbstract:The vortex-induced vibration (VIV) of an elastically mounted rotating circular cylinder vibrating in a uniform flow is studied numerically. The cylinder is allowed to vibrate only in the cross-flow direction. In the numerical simulations, the Reynolds number, the mass ratio, and the damping ratio are kept constants to 500, 11.5, and 0, respectively. Simulations are performed for rotation rates of α = 0, 0.5, and 1 and a range of Reduced velocities from 1 to 13, which covers the entire lock-in regime. It is found that the lock-in regime of a rotating cylinder is wider than that of a non-rotating cylinder for α = 0, 0.5, and 1. The vortex shedding pattern of a rotating cylinder is found to be similar to that of a non-rotating cylinder. Next, simulations are performed for three typical Reduced velocities inside the lock-in regime and a range of higher rotation rates from α = 1.5 to 3.5 to investigate the effect of the rotation rate on the suppression of VIV. It is found that the VIV is suppressed when the rotation rate exceeds a critical value, which is dependent on the Reduced Velocity. For a constant Reduced Velocity, the amplitude of the vibration is found to increase with increasing rotation rate until the latter reaches its critical value for VIV suppression, beyond which the vibration amplitude becomes extremely small. If the rotation rate is greater than its critical value, vortex shedding ceases and hairpin vortices are observed due to the rotation of the cylinder.
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numerical investigation of the effect of plane boundary on two degree of freedom of vortex induced vibration of a circular cylinder in oscillatory flow
Ocean Engineering, 2018Co-Authors: Adnan Munir, Ming Zhao, Dezhi NingAbstract:Abstract The laying of subsea pipelines often produces situations where the pipeline is suspended above the seabed due to local erosion of sediment. In this paper, flow induced vibration of a circular cylinder close to a plane boundary in an oscillatory flow is studied through two-dimensional numerical simulations. The circular cylinder and the plane boundary represents a pipeline and the seabed, respectively. It is found that the plane boundary affects the vibration amplitude in the cross-flow direction significantly. The vibration in the vertical direction ceases if Reduced Velocity exceeds 6 for KC = 5 and 12 for KC = 10, respectively. The vibration in the cross-flow direction stops when the Reduced Velocity exceeds a critical value because the effective KC number and the effective Reduced Velocity, which are both based on the relative Velocity of the cylinder to the fluid motion, are extremely small. For KC = 10, the vortex shedding is found to be in one pair regime for most of the Reduced velocities and non-vortex shedding regime exists at large Reduced velocities. Because the shear layers generated from the plane boundary attract the vortices generated from the cylinder, vortex shedding occurs only at the bottom side of the cylinder.
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three dimensional numerical simulation of vortex induced vibration of an elastically mounted rigid circular cylinder in steady current
Journal of Fluids and Structures, 2014Co-Authors: Ming Zhao, Liang ChengAbstract:Abstract Vortex-induced vibration (VIV) of an elastically mounted rigid circular cylinder in steady current is investigated by solving the three-dimensional Navier–Stokes equations. The cylinder is allowed to vibrate only in the cross-flow direction. The aim of this study is to investigate the variation of the vortex shedding flow in the axial direction of the cylinder and to study the transition of the flow from two-dimensional (2D) to three-dimensional (3D) for VIV of a cylinder. Simulations are carried out for a constant mass ratio of 2, the Reynolds numbers ranging from 150 to 1000 and the Reduced velocities ranging from 2 to 12. The three-dimensionality of the flow is found to be the strongest in the upper branch of the VIV response and weakest in the initial branch. The 2S and 2P vortex shedding modes are found to coexist along the cylinder span in the upper branch, leading to strong variations of the lift coefficient in the axial direction of the cylinder. The difference between the flow transition from 2D to 3D in the VIV lock-in regime and that in the wake of a stationary cylinder is identified. The transition mode B found in the wake of a stationary cylinder is also found in the wake of a vibrating cylinder. The critical Reynolds number for flow transition from 2D to 3D of a cylinder undergoing cross-flow VIV at a Reduced Velocity of 6 is found to be greater than that for a stationary cylinder. For a constant Reduced Velocity of 6, the wake flow changes from 2D to 3D as the Reynolds number is increased from 250 to 300. Some 2D numerical simulations are performed and it is found that the 2D Navier–Stokes (NS) equations are not able to predict the VIV in the turbulent flow regime, while the 2D Reynolds-averaged Navier–Stokes (RANS) equations improve the results.
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vortex induced vibrations of a rotating circular cylinder at low reynolds number
Physics of Fluids, 2014Co-Authors: Ming Zhao, Liang ChengAbstract:Vortex-induced vibration (VIV) of a rotating circular cylinder at a low Reynolds number of 150 and a low mass ratio of 2 is studied numerically. Simulations are conducted at three rotation rates of α = 0, 0.5, and 1 and Reduced velocities in the range of 1–13 with an interval of 0.2. The numerical results show that the rotation of the cylinder increases the response amplitude and widens the lock-in regime for the one-degree-of-freedom (1-dof) VIV in the cross-flow direction. The two-degree-of-freedom (2-dof) responses of the cylinder at α = 0.5 and 1 are significantly different from that at α = 0. For the 2-dof VIV, the response amplitude in the inline direction, which is much smaller than that in the cross-flow direction at α = 0, is increased significantly at α = 0.5 and 1. One initial branch is found at α = 0.5 and two initial branches are found at α = 1. In the initial branches, the response frequency locks onto a frequency that is smaller than the natural frequency of the cylinder and the response amplitude increases with the Reduced Velocity. The vortex shedding is found to be in the P+S mode for Reduced velocities near the higher boundary of the initial branches and 2S mode in all other Reduced Velocity ranges for the 2-dof VIV. Simulations are conducted under both the increasing and decreasing Reduced Velocity conditions. A hysteresis region is found near the higher boundary of the lower branch for α = 0, 0.5, and 1 in the 1-dof of VIV and for α = 0 in the 2-dof VIV. The hysteresis region occurs near the higher boundary of the initial branches for α = 0.5 and 1 in the 2-dof VIV. By analysing the component of the force coefficient that is in phase with the Velocity of the cylinder, it is found that pressure force excites the vibration and the viscous force damps the vibration in both the inline and the cross-flow directions in the 2-dof VIV. The magnitude of the time averaged pressure and viscous force coefficients that are in phase with the velocities of the cylinder in the lock-in regime are found to be much greater than their counterparts outside the lock-in regime.
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Flow induced vibration of two rigidly coupled circular cylinders in tandem and side-by-side arrangements at a low Reynolds number of 150
Physics of Fluids, 2013Co-Authors: Ming ZhaoAbstract:Flow induced vibration of two rigidly coupled identical circular cylinders in tandem and side-by-side arrangements at a low Reynolds number of 150 is studied numerically. The two cylinders vibrate in the cross-flow direction and have the same displacement. The Navier-Stokes equations are solved by the finite element method and the equation of motion of the cylinders is solved by the fourth-order Runge-Kutta algorithm. Simulations are conducted for a constant mass ratio of 2 and the gap ratios (defined as the ratio of the centre-to-centre distance between the two cylinders L to the cylinder diameter D) of 1.5, 2, 4, and 6. The Reduced velocities range from 0.5 to 15 with an increment of 0.5 for the tandem arrangement and from 0.5 to 30 with an increment of 0.5 for the side-by-side arrangement. It is found that the gap between the two cylinders has significant effect on the response. For a tandem arrangement, the lock-in regime of the Reduced Velocity is narrower than that of a single cylinder for L/D = 1.5...
Liang Cheng - One of the best experts on this subject based on the ideXlab platform.
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three dimensional numerical simulation of vortex induced vibration of an elastically mounted rigid circular cylinder in steady current
Journal of Fluids and Structures, 2014Co-Authors: Ming Zhao, Liang ChengAbstract:Abstract Vortex-induced vibration (VIV) of an elastically mounted rigid circular cylinder in steady current is investigated by solving the three-dimensional Navier–Stokes equations. The cylinder is allowed to vibrate only in the cross-flow direction. The aim of this study is to investigate the variation of the vortex shedding flow in the axial direction of the cylinder and to study the transition of the flow from two-dimensional (2D) to three-dimensional (3D) for VIV of a cylinder. Simulations are carried out for a constant mass ratio of 2, the Reynolds numbers ranging from 150 to 1000 and the Reduced velocities ranging from 2 to 12. The three-dimensionality of the flow is found to be the strongest in the upper branch of the VIV response and weakest in the initial branch. The 2S and 2P vortex shedding modes are found to coexist along the cylinder span in the upper branch, leading to strong variations of the lift coefficient in the axial direction of the cylinder. The difference between the flow transition from 2D to 3D in the VIV lock-in regime and that in the wake of a stationary cylinder is identified. The transition mode B found in the wake of a stationary cylinder is also found in the wake of a vibrating cylinder. The critical Reynolds number for flow transition from 2D to 3D of a cylinder undergoing cross-flow VIV at a Reduced Velocity of 6 is found to be greater than that for a stationary cylinder. For a constant Reduced Velocity of 6, the wake flow changes from 2D to 3D as the Reynolds number is increased from 250 to 300. Some 2D numerical simulations are performed and it is found that the 2D Navier–Stokes (NS) equations are not able to predict the VIV in the turbulent flow regime, while the 2D Reynolds-averaged Navier–Stokes (RANS) equations improve the results.
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vortex induced vibrations of a rotating circular cylinder at low reynolds number
Physics of Fluids, 2014Co-Authors: Ming Zhao, Liang ChengAbstract:Vortex-induced vibration (VIV) of a rotating circular cylinder at a low Reynolds number of 150 and a low mass ratio of 2 is studied numerically. Simulations are conducted at three rotation rates of α = 0, 0.5, and 1 and Reduced velocities in the range of 1–13 with an interval of 0.2. The numerical results show that the rotation of the cylinder increases the response amplitude and widens the lock-in regime for the one-degree-of-freedom (1-dof) VIV in the cross-flow direction. The two-degree-of-freedom (2-dof) responses of the cylinder at α = 0.5 and 1 are significantly different from that at α = 0. For the 2-dof VIV, the response amplitude in the inline direction, which is much smaller than that in the cross-flow direction at α = 0, is increased significantly at α = 0.5 and 1. One initial branch is found at α = 0.5 and two initial branches are found at α = 1. In the initial branches, the response frequency locks onto a frequency that is smaller than the natural frequency of the cylinder and the response amplitude increases with the Reduced Velocity. The vortex shedding is found to be in the P+S mode for Reduced velocities near the higher boundary of the initial branches and 2S mode in all other Reduced Velocity ranges for the 2-dof VIV. Simulations are conducted under both the increasing and decreasing Reduced Velocity conditions. A hysteresis region is found near the higher boundary of the lower branch for α = 0, 0.5, and 1 in the 1-dof of VIV and for α = 0 in the 2-dof VIV. The hysteresis region occurs near the higher boundary of the initial branches for α = 0.5 and 1 in the 2-dof VIV. By analysing the component of the force coefficient that is in phase with the Velocity of the cylinder, it is found that pressure force excites the vibration and the viscous force damps the vibration in both the inline and the cross-flow directions in the 2-dof VIV. The magnitude of the time averaged pressure and viscous force coefficients that are in phase with the velocities of the cylinder in the lock-in regime are found to be much greater than their counterparts outside the lock-in regime.
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numerical investigation of vortex induced vibration of a circular cylinder in transverse direction in oscillatory flow
Ocean Engineering, 2012Co-Authors: Ming Zhao, Liang ChengAbstract:Abstract One-degree-of-freedom (1DOF) vortex-induced vibration (VIV) of a circular cylinder in oscillatory flow is investigated numerically. The vibration of the cylinder is confined in the cross-flow direction only. Reynolds-Averaged Navier–Stokes equations and k–ω turbulent equations are solved by a Petrov–Galerkin finite element method. Simulations are carried out for two Keulegan–Carpenter ( KC ) numbers of 10 and 20 and Reduced velocities ranging from 1 to 36. It is found that the response contains only one frequency component as Reduced Velocity is less than 8 for both KC numbers and contains multiple frequency components as Reduced Velocity exceeds 8. All the frequency components are multiples of the frequency of the oscillatory flow except at a few Reduced velocities. For KC =20, the vibration frequency components (or vibration mode) change frequently as Reduced Velocity is larger than 10. Wavelet transform is applied to analyse instant frequency components at a specific time instant. It was found that the change from one vibration mode to another is regular and periodic. Based on the wavelet transformation, a mode-averaging technique is proposed to identify all the frequency components that ever occurred in the vibration. The variation of amplitudes and frequencies of the vibration with Reduced Velocity is studied.
Norsarahaida Amin - One of the best experts on this subject based on the ideXlab platform.
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Unsteady boundary layer flow of a micropolar fluid near the rear stagnation point of a plane surface
International Journal of Thermal Sciences, 2003Co-Authors: Yian Yian Lok, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the rear stagnation point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the Reduced Velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in table
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unsteady boundary layer flow of a micropolar fluid near the forward stagnation point of a plane surface
International Journal of Engineering Science, 2003Co-Authors: Yian Yian Lok, P Phang, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the rear stagnation point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the Reduced Velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in tables
Fotis Sotiropoulos - One of the best experts on this subject based on the ideXlab platform.
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Vortex-induced vibrations of an elastically mounted sphere: The effects of Reynolds number and Reduced Velocity
Journal of Fluids and Structures, 2016Co-Authors: Suresh Behara, Fotis SotiropoulosAbstract:Abstract The dynamics and wake modes of vortex-induced vibrations of a sphere mounted on elastic supports in all three spatial directions are systematically investigated via numerical simulations. In our previous work ( Behara et al. 2011 ), we studied this problem for a sphere of m * = 2 at Re=300 and Reduced Velocity in the range of U * = 0 – 9 . We showed that for 4.8 ≤ U * ≤ 9 the sphere exhibits two distinct synchronized oscillation regimes each associated with a distinct wake mode: the hairpin mode (hairpin vortices shed in the wake) and the spiral mode (intertwined vortices shed in the wake). Here we contribute new physical insights into the dynamics of the elastically mounted sphere by probing vibration modes and vortex shedding patterns for a greater range of Reduced velocities ( U * = 0 − 13 ) at Re=300 and for the range of Reynolds numbers 300 ≤ Re ≤ 1000 for a fixed Reduced Velocity U * = 9 . For the Re=300 case the lock-in regime is observed for 5.8 ≤ U * ≤ 12.2 with the spiral mode, while the lock-in region with the hairpin mode exists only for 4.8 ≤ U * ≤ 8 . The hairpin mode is found to become unstable and merge with the response branch of the spiral mode at U * = 9 . In the lock-in regime corresponding to the spiral shedding mode the vibrating sphere moves along a circular orbit on the transverse plane. The hairpin shedding mode reappears in the wake after the synchronization region corresponding to the spiral mode ends. Varying Reynolds number ( 300 ≤ Re ≤ 1000 ) for fixed Reduced Velocity ( U * = 9 ), we find that the synchronized oscillations persist up to Re=1000, but the wake mode and sphere trajectories depend strongly on Reynolds number. The sphere sheds spiral vortices up to Re ∼ 500 , but the wake transitions from the spiral to the hairpin shedding mode in the range of Re = 500 – 600 . During this wake transition the sphere trajectory on the transverse plane changes from circular to elliptical orbits. The sphere exhibits periodic oscillations in the spiral mode for Re ≤ 500 , whereas for Re ≥ 600 , when the wake is in the hairpin mode, oscillations become non-stationary.
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vortex induced vibrations of an elastically mounted sphere with three degrees of freedom at re 300 hysteresis and vortex shedding modes
Journal of Fluid Mechanics, 2011Co-Authors: Suresh Behara, Iman Borazjani, Fotis SotiropoulosAbstract:Fluid–structure interaction (FSI) simulations are carried out to investigate vortex-induced vibrations of a sphere, mounted on elastic supports in all three spatial directions. The Reduced Velocity ( ) is systematically varied in the range , while the Reynolds number and Reduced mass are held fixed at and , respectively. In the lock-in regime, two distinct branches are observed in the response curve, each corresponding to a distinct type of vortex shedding, namely, hairpin and spiral vortices. While shedding of hairpin vortices has been observed in several previous investigations of stationary and vibrating spheres, the shedding of intertwined, longitudinal spiral vortices in the wake of a vibrating sphere is reported herein for the first time. When the wake is in the hairpin shedding mode, the sphere moves along a linear path in the transverse plane, while when spiral vortices are shed, the sphere vibrates along a circular orbit. In the spiral mode branch, the simulations reveal hysteresis in the response amplitude at the beginning of the lock-in regime. Lower-amplitude vibrations are found as the sphere sheds hairpin vortices for increasing up until the beginning of the synchronization regime. On the other hand, higher-amplitude oscillations persist for the spiral mode as is decreased from the point of the start of the synchronization. The hairpin mode is found to be unstable for the value of Reduced Velocity where the spiral and hairpin solution branches merge together. When this point is approached along the hairpin solution branch, the sphere naturally transitions from shedding hairpin vortices and moving along a linear path to shedding spiral vortices and moving along a circular path in the transverse plane. The spiral mode was not observed in the work of Horowitz & Williamson ( J. Fluid Mech. , vol. 651, 2010, pp. 251–294), who studied experimentally the vibration modes of a freely rising or falling sphere and only reported zigzag vibrations. Our results suggest that this apparent discrepancy between experiments and simulations should be attributed to the fact that, for the range of governing parameters considered in the simulations, the elastic supports act to suppress streamwise vibrations, thus subjecting the sphere to a nearly axisymmetric elasticity constraint and enabling it to vibrate transversely along a circular path.